Chemistry Reference
In-Depth Information
6.7.5
Dyeing-process behaviour with and without
sensor system
In this section, the use of a wall-jet electrode (with optimal values for its
parameters as described in section 6.7.2) and the method to detect simulta-
neously sodium dithionite, sulphite and indigo (see section 6.7.4) are eval-
uated as a function of reproducible dyeing processing. In order to evaluate
this, a spectrophotometric method was used to measure the amount of dye
absorbed and/or adsorbed by the dyed fabric.
The first section of Fig. 6.17 and Fig. 6.18 shows the changes of indigo and
sodium dithionite concentration (measured in the bath solution with the
developed sensor) and of absorbance and wavelength of the dye taken up
by the fabric (measured off-line, spectrophotometrically), respectively. The
experiment was performed in a continuous dyeing line with a similar step
as the one for bleaching described in Chapter 5, section 5.4.2. The data of
Fig. 6.18 show that the concentration of dye stuff decreases as a function of
time, which can easily be understood as the uptake of indigo by the fabric
without a considerable change in volume of the bath solution. This means
that dilution of indigo occurs measured as a concentration decrease. It can
be seen that the concentration of dithionite also decreases. This can be
explained by the fact that, besides uptake by the fabric, dithionite is also
consumed in the bath. Convection in the bath causes uptake of oxygen in
the bath solution that can oxidise dithionite and indigo easily.
The second sections of Fig. 6.17 and Fig. 6.18 represent the same para-
meters as in the first section, but this time the concentrations of indigo and
sodium dithionite are not only measured but are also row controlled. This
6.16
Variation with time of amperometric currents, resulting
concentrations and concentrations obtained by titration of indigo,
dithionite and sulphite, obtained from a cell solution in which the
concentrations of dithionite and indigo were increased
periodically by recurrent additions. (a) Variation of the
experimental currents measured at a platinum wall-jet electrode
with a flowrate of 1.5 l min
-
1
, a diameter of the electrode of
3.0 mm, a diameter of the nozzle of 2.0 mm and a NES gap of
2.5 mm and at a potential of (1)
-
0.55 V, (2) 0.45 V and (3) 0.8 V
vs. Ag|AgCl. (b) Variation of the concentrations of indigo (
¥
),
dithionite () and sulphite (), obtained by titration and from
limiting-currents (continuous lines) measured at a platinum wall-
jet electrode with a flow rate of 1.5 l min
-
1
, a diameter of the
electrode of 3.0 mm, a diameter of the nozzle of 2.0 mm and a
NES gap of 2.5 mm and by using Equations 6.29-6.31 (full lines).
(Reprinted from
Analytica Chimica Acta
, Vol 486, No 1, Gasana
et al
., 'A wall-jet disc . . .' pp 73-83, Copyright 2003, with
permission from Elsevier.)
